This invention relates to an apparatus for forming a visible image by chromogenic reaction of a colorless dye encapsulated in microcapsules and more particularly to such an image formation apparatus provided with supplemental light source for controlling the quality of produced image.
Japanese Patent Publication Tokkai Nos. 58-88739 and 59-149343 have disclosed a light and pressure sensitive image formation material composed of a light-accepting sheet having a base sheet coated with microcapsules encapsulating a photosensitive substance which hardens upon exposure to light and a colorless dye and an image transfer sheet coated with a developer material having chromogenic effects on the colorless dye. To briefly explain the method of image formation by using such a light and pressure sensitive material, an image-forming beam of light reflected, for example, from an original document is made incident on the light-accepting sheet to selectively harden the photosensitive substance encapsulated in those of the microcapsules exposed to light. If the image transfer sheet is thereafter superposed on the light-accepting sheet and pressed together between rollers, for example, those of the microcapsules not exposed to light and not hardened thereby are ruptured and allow the encapsulated colorless dye to flow out and react chromogenically with the developer material on the image transfer sheet.
As a material which hardens upon exposure to light, use is generally made of a monomer or cross-linking (bridge-forming) macromolecules with a polarization initiator. A polymerization initiator serves to respond to light in a specified range of wavelength to form radicals and to thereby initiate polymerization of a monomer or cross-linking combination of macromolecules. If so-defined Y capsules containing a photosensitive material which hardens upon exposure to blue (B) light and a colorless dye which turns yellow (Y), M capsules containing a photosensitive material which hardens upon exposure to green (G) light and a colorless dye which turns magenta (M), and C capsules containing a photosensitive material which hardens upon exposure to red (R) light and a colorless dye which turns cyanic (C) are uniformly distributed to form a light-accepting sheet and a white image-forming beam of light is made incident thereon, a color image can be formed by the aforementioned technology.
If a light-accepting sheet thus formed is used with a prior art image formation apparatus, however, a faithfully reproduced color image cannot always be obtained because the different photosensitive materials encapsulated in different types of capsules have different photosensitivity characteristics.
Photosensitivity characteristics include the so-called gamma (change in exposure/change in image density) which represents the dynamic range and sensitivity. FIG. 1 shows, for example, the relationship between image density and exposure for each of the three colors Y, M and C from Y, M and C capsules exposed respectively to B, G and R light by a prior art image formation apparatus. Since the gamma of Y capsules is extremely small and their sensitivity is somewhat high, Y comes out relatively strongly in high density areas but weakly in low density areas. This means that, when a produced full-color image is looked at as a whole, high density areas look greenish and low density areas look bluish.
It has been known to use filters to adjust sensitivity. For the situation discussed above, for example, a filter for absorbing C light may be used to control the spectroscopic energy distribution of the image-forming light, but gammas of Y, M and C capsules cannot be changed by filters alone. As a result, high density and low density areas tend to appear differently colored.
A method of controlling the gamma of photosensitive microcapsules has been disclosed, for example, in Japanese Patent Publication Tokkai No. 59-149343. In general, oxygen in air penetrates the resin shells of the microcapsules and the interior of the shells is in a saturated condition. When such photosensitive capsules are exposed to light, a part of the light energy is expended in consuming oxygen and the hardening takes place only after oxygen is consumed. In other words, oxygen serves to impede the hardening of the capsules by light. If such capsules are supplementally exposed to light, therefore, oxygen is additionally consumed and the hardening can be accelerated, or the gamma can be adjusted to a certain extent.